Drilling, completion, and production of reservoir wells involve monitoring of various subsurface formation parameters. For example, parameters such as reservoir pressure and permeability of the reservoir rock formation are often measured to evaluate a subsurface formation. Fluid may be drawn from the formation and captured to measure and analyze various fluid properties of a fluid sample. Monitoring of such subsurface formation parameters can be used, for example, to determine formation pressure changes along the well trajectory or to predict the production capacity and lifetime of a subsurface formation.
Some known downhole measurement systems may obtain these parameters through wireline logging via a formation tester or sampling tool. Alternatively, a formation tester or sampling tool may be coupled to a drill string in-line with a drill bit (e.g., as part of a bottom hole assembly) and a directional drilling subassembly. Such formation testing or sampling tools may be implemented using fluid sampling probes, each of which has a one or more nozzles, inlets, or openings into which formation fluid may be drawn. A variety of types of sampling tools or probes are currently used to extract formation fluid. For example, some sampling tools use an extendable probe, which is sometimes generally referred to as a packer, having a single nozzle or inlet to draw formation fluid. The probe (e.g., the nozzle or inlet), is typically surrounded by a circular or ring-shaped rubber interface or packer that is extended toward and forced against a borehole wall to sealingly engage the nozzle or inlet with a subterranean formation. In some cases, the seal provided by a packer may be implemented using an inflatable packer device such as, for example that described in U.S. Pat. No. 6,301,959. Some sampling probes or packers provide multiple inlets (e.g., two inlets) where at least one inlet is a sample inlet and at least one other inlet is a guard inlet. However, in the case of a multi-inlet configuration, multiple packers may be used such that at least one packer includes a sample inlet and another separate packer or packers include the guard inlet or inlets.
In operation, a sampling probe or packer may be extended via hydraulics from the downhole tool to drive its nozzle or inlet against the borehole wall adjacent a portion of the formation to be evaluated. A pumpout assembly is then activated to draw fluid from the formation into the probe and to convey the formation fluid to a downhole testing device and/or a sample collection vessel that can be retrieved to the surface to enable laboratory analysis of the sample fluid contained therein. Additionally, as noted above, the sampling probe inlet is typically surrounded by a packer that facilitates the sealing of the sampling probe inlet against the borehole wall and, thus, facilitates the application of a pressure to the formation to efficiently draw fluid from the formation.
When drawing fluid from a formation, a certain amount of filtrate can also be drawn into the probe along with the formation fluid, thereby contaminating the sample fluid. The degree of contamination (e.g., the percent contamination) in the sample fluid is initially relatively large, but typically decreases over time as the sampling probe continues to draw formation fluid from the formation. Thus, fluid extracted from the formation by the sampling probe is usually discarded until, at some time during the sampling process, the level of contamination is sufficiently low to permit capture of a sample having an acceptable purity for testing or evaluation purposes.
With single inlet sampling probes (i.e., a sampling probe providing only a sample inlet and no guard inlet), a relatively large amount of fluid may have to be drawn from the formation before an acceptable purity or contamination level is achieved. However, to draw such a large amount of fluid may require a significant amount of time, which can be costly, particularly if the job is delayed by the sampling process. Additionally, while the level of contamination can be reduced significantly by first drawing a large amount of fluid from the formation, the minimum level or degree of contamination achievable with a single inlet probe may remain high enough to affect the accuracy of the test results.
While single inlet sampling probes have proven to be relatively effective, dual inlet or guard probes can provide improved, focused sampling of formation fluids. Such dual inlet or guard probes typically include concentric nozzles or inlets, where a central nozzle or inlet is configured to act as the sampling inlet and an outer nozzle or inlet is configured to act as a guard inlet. More specifically, the guard inlet, which forms a perimeter or ring around the central or sampling inlet, is configured to draw substantially all of the filtrate away from the central part of the probe and, thus, the central inlet, thereby enabling the central or sampling inlet to draw in formation fluid that is relatively free of contamination (e.g., filtrate). Dual inlet or guard probes also utilize two packers to seal the probe against the formation to be evaluated. An outer packer surrounds the guard nozzle or inlet and an inner packer surrounds the central sample nozzle or inlet in the area between an outer wall of the sample inlet and an inner wall of the guard inlet.
In contrast to single inlet probes, dual inlet of guard probes can significantly reduce the time required to achieve a sufficiently low level of sample contamination (i.e., a reduced sample cleanup time), which can significantly decrease costs associated with evaluation of a formation (e.g., reduced station times). Additionally, dual inlet or guard probes can also provide significantly improved sample purity (i.e., a lower level of contamination) than possible with conventional single inlet probes. Such an increased level of sample purity can provide more accurate information for optimizing completion and production decisions.
Although dual inlet or guard probes have enabled significantly reduced sample cleanup times and improved sample purity levels, such dual inlet probes can introduce certain operational complexities or difficulties. In particular, each nozzle or inlet typically has its own independently controlled pumpout and flowlines (e.g., guard and sample flowlines), which makes it difficult to control precisely the relative pumping rates (i.e., the pumping distribution) of the sample and guard nozzles or inlets and flowlines. An inability to control precisely the relative pumping rates of the guard and sample inlets and flowlines can lead to higher levels of contamination in the sample fluid, compromising of the inner packer seal or breakage of the inner packer, longer sample cleanup times, etc. Further, the use of an independent pumpout for each inlet and flowline results in less available power for each pumpout and can also result in a lower overall power efficiency.
With some known dual inlet or guard probe systems, the differential pressure developed across the pumpouts is relatively fixed based primarily on the configuration of the displacement units within the pumpouts and the mobility of the fluid to be sampled. Thus, for a particular fluid mobility, a particular displacement unit may be selected to provide a desired pumping rate for each of the guard and sample inlets and flowlines as well as a relative pumping rate or pumping distribution between the guard and sample systems. However, fluid mobility may not be known precisely prior to sampling and, thus, a selected displacement unit may develop a differential pressure that results in poor fluid sampling (e.g., flow between the sample and guard inlets and, thus, increased sample contamination) and/or compromise of or damage to the inner packer. Additionally, further adjustments of the pumping rate and differential pressure developed by the pumpout(s) typically requires replacement of the displacement unit(s) at the surface, which is time consuming and costly.